1. Field of the Invention
This invention relates to rolling, and more particularly to descaling a strip by abrasive powder.
2. Description of the Prior Art
There is known a method of abrasive powder descaling of a strip (SU, A, 954,131) in which the strip is first treated with the abrasive powder by applying to the powder and maintaining of a pressure between 4 and 6 MPa, the strip being pulled through by applying thereto a tension force. The strip is then pickled. However, ecological hazard makes the method less promising.
A method bearing the closest resemblance to one to be hereinafter described resides in that the strip is pulled at a tension through at least two successive descaling zones where scale is removed by applying to the strip a pressure of abrasive powder varying within a range preset for each descaling zone (Bulletin "Chermetinformatsia", 1987, No. 6, pp. 42 to 45). This prototype method is characterized by the following disadvantages. Firstly, monitoring the strip tension, depending largely on the resistance to pulling the strip through the descaling zone occupied by the abrasive powder, is not envisaged. This makes the strip more susceptible to breaking due to high tension forces necessary for overcoming the resistance on the part of the abrasive powder pressed to the strip. Each strip breaking causes loss of metal, unscheduled stops for discharging the powder, threading and welding together strip ends, and recharging the powder. In addition, a sudden increase in strip tension at the inlet to the descalling zone arising in response to a sudden drop in the pressure of powder on the strip makes it imperative to operatively change operation of the strip tension unit acting to pull the strip through the preceding descaling zone. Unfortunately, this is impossible in the absence of facilities for controlling the strip tension at the inlet to the descaling zone.
The known method also fails to control the force of pressure applied by the powder to the strip and the tension of the strip to attain the final aim of the process, viz., thorough cleaning of the strip from scale, which leads to high power consumption as it is impossible to preset a minimum powder pressure necessary for removing scale from each specific strip.
It is not advisable, when using this prior art method, to increase the pressure of powder on the strip to above the maximum working pressure, since such an increase fails to reduce the quantity of residual scale at the strip surface, and makes the strip surface scratched. For example, in the first descaling zone the working pressure rage of powder applied to the strip is 1.5-2.0 MPa. However, for a given strip (taking into account the natural spread in the physical and mechanical properties of strips even within one run) the maximum allowable pressure can be 1.8 or 1.9 MPa, rather than 2.0 MPa. Therewith, the quantity of residual scale at the strip surface is minimal at this pressure (such as 30% of the initial scale present at the strip surface), whereas an increase in this pressure to 2.0 MPa leads to strip surface defects and unjustified power consumption rather than to a reduction in the quantity of residual scale at the strip surface.
The aforedescribed makes the prior art method disadvantageous for controlling an advanced and highly efficient apparatus for descaling strips by abrasive powder, preventing strips from breaking, and attaining thorough scale removal from strips.